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Our ability to reason by analogy has been claimed to be at the "core of cognition" (Hofstadter, 2000). Two objects, concepts, or even real-world situations are analogous if they are dissimilar on the surface but share some higher-order, structural similarities. Although analogical reasoning has been the focus of many behavioral and computational investigations in the past three decades, not much is known about analogy's neural basis.

Among functional magnetic resonance imaging (fMRI) studies that have examined analogical reasoning, most point to the rostrolateral prefrontal cortex (RLPFC) as playing a pivotal role in mediating analogy (e.g., Cho et al., 2010). This brain area has been hypothesized to support the ability to simultaneously compare and integrate multiple relations between things, termed "relational integration", a necessary part of successful analogy-making (e.g., Gentner, 1983).

However, RLPFC activity is also observed during a wide range of non-analogical cognitive tasks, including episodic memory retrieval (e.g., Ranganath et al., 2000) and multi-tasking (e.g., Koechlin et al., 1999), leading to the alternative hypothesis that this brain area coordinates the outcomes of subgoals in the service of fulfilling a main goal (e.g., Koechlin et al., 1999). On this view, relational integration during analogy is just a special case of coordinating subgoals. In this study, we compared visuospatial analogical reasoning to another complex task that required the coordination of subgoals but not the integration of relations, specifically, in order to test the specificity of RLPFC for analogy.

We asked participants in the fMRI scanner to solve visuospatial analogies and a non-analogical task that was similarly difficult and perceptually-matched to the analogies. In the "analogy" condition (Figure 1A & 1B), participants had to consider the spatial relations between the colored shapes in the source set (top) and select the response below with the same set of relations. On the other hand, in the "item" condition (Figure 1C & 1D), the relationships between the colored shapes were not relevant: the correct answer was arrived at by selecting the choice that contains the same three colored shapes as the source, in any order. While both reasoning tasks required evaluation of multiple subgoals to select an answer, only the analogy condition required integration of relations (here, relations in space).

Figure 1: Stimuli from the analogy (A & B) and item tasks (C & D).

We were also interested in the role of the parietal lobe in analogy given that inferior and superior parietal cortex participate in representing categorical spatial relations (Amorapanth et al., 2010). Similarly, during visuospatial analogical reasoning, it is necessary to extract the spatial relations between the shapes involved. Finally, the need to inhibit incorrect responses based on surface, not relational, features has been proposed to be a central part of the analogy process (e.g., Robin and Holyoak, 1995), so we predicted greater activity during analogy in inferior frontal gyri (IFG), areas of the brain often implicated by inhibitory control (e.g., Aron et al., 2003).

With region-of-interest (ROI) fMRI analyses, we found that both left and right RLPFC were more active during the analogy task relative to the item task (Figure 2A). Because the analogy task alone required integration of relations, these results suggest that RLPFC activity is more strongly driven by the need to integrate relational information rather than coordinate subgoal outcomes, more generally. However, given this area’s role in tasks other than analogical reasoning, another explanation is that increasingly abstract information preferentially recruits RLPFC, even when task or stimulus complexity is held constant (e.g., Christoff and Keramatian, 2007). Here, successful analogy-making required extracting an abstract pattern of relations – a pattern that did not depend on the specific perceptual features of the stimuli. Thus, greater RLPFC activity observed during the analogy condition could be due to the more abstract nature of the information relevant to the task.

Figure 2: Region-of-interest analyses for analogy > item contrast in

A) RLPFC, B) inferior frontal gyrus, and C) inferior parietal cortex.

We also found greater activity in bilateral inferior parietal cortex ROIs during analogy, a task in which spatial relations were informative, relative to a task in which spatial relations were irrelevant (the item task) (Figure 2C). These results suggest that the extraction of the relative spatial locations of multiple objects is a critical component of the analogical process when analogies are in the visuospatial domain. In verbal analogies, relations between concepts may instead recruit areas of the brain that represent semantic knowledge, such as the temporal lobes.

Finally, in both whole-brain (Figure 3) and ROI analyses (Figure 2B), we observed greater activity in right IFG during analogy relative to the item task (and, in ROI analyses only, in left IFG). Bilateral IFG have been implicated in inhibitory control: in the right hemisphere, by the inhibition of a dominant response (e.g., Aron et al., 2003), and in the left, by the need to resolve conflict among competing semantic representations (e.g., Thompson-Schill et al., 1997). Thus, IFG may be engaged during analogy when it is necessary to ignore irrelevant aspects of the stimuli in order to identify relational similarity or to inhibit prepotent responses based on superficial, not relational, dimensions.

Figure 3: Whole-brain analysis of analogy > item.

In sum, we found evidence that bilateral RLPFC is more strongly engaged during the integration of relations in analogy relative to the coordination of subgoal outcomes, more generally, even when the non-analogical task was more difficult for participants to solve. Additionally, the neural basis of analogical reasoning lies not only in the anterior aspects of prefrontal cortex, but also in areas of the brain involved in inhibition or cognitive control and, for visuospatial analogies, areas that represent spatial relation knowledge. Together, these areas work in concert during analogical reasoning to extract and integrate relational information while suppressing the tendency to think superficially rather than structurally.